A company wishing to provide telephone service to the members of the company may utilize a private branch exchange (PBX). Each telephone set that connects to and is served by the PBX is referred to as a client station or station. The use of a PBX may help to avoid the burden and cost of separately connecting each of the company's telephone sets to the public switched telephone network (PSTN). In addition, a PBX may provide additional advanced features which may not be achievable by connecting the stations directly to the PSTN. For example, the PBX may provide improved privacy when calling between stations, since conventional calls on the PSTN are transmitted across a public network, which is subject to eavesdropping. In addition, the PBX may provide additional services, such as call park, call pickup, call transfer, and call forward to other stations. Voice Over Internet Protocol (VoIP) has seen increased widespread usage. An IP PBX is a type of PBX that connects to client stations on the private side via an IP network, and connects to an Internet Telephone Service Provider (ITSP) on the public side via an IP network. The ITSP includes PSTN gateways, which provide PSTN termination services. Voicemail services for the client stations may also be provided by the ITSP or from a separate voicemail service provider, such as an Internet Voice Mail Service Provider (IVMSP).
A client station that is assigned a voice mailbox is notified by the voicemail server when there is a change in the status of its mailbox. The station may then turn its message waiting indicator (MWI) lamp on or off, depending on the latest status of the mailbox. Using Session Initiation Protocol (SIP) as the signaling protocol, a conventional implementation of voicemail functionality requires that each station subscribe, either explicitly or implicitly, to the message-summary event notification from the voicemail server. This subscription establishes a binding of the mailbox to that particular PBX station. The binding includes such information as the ID of the mailbox and the IP address and port number that the voicemail server will use to contact the client station. If the IP PBX supports multiple stations, each with a different mailbox ID, then each station shall create a separate subscription with the voicemail server. As a result, the service provider (SP) supporting the voicemail server will maintain a separate subscription for each mailbox, each with a different User ID and password. All of this increases the administrative burden and cost associated with providing voice mailboxes to the client stations.